libstdc++: Clean up @diff@ markup in some I/O tests

We have a number of 27_io/* tests with comments like this:

// @require@ %-*.tst
// @diff@ %-*.tst %-*.txt

It seems that these declare required data files used by the test and a
post-test action to compare the test output with the expected result.
We do have tests that depend on some *.tst and/or *.txt files that are
copied from testsuite/data into each test's working directory before it
runs, so the comments are related to those dependencies.  However,
nothing in the current test framework actually makes use of these
comments. Currently, every test gets a fresh copy of every *.tst and
*.txt file in the testsuite/data directory, whether the test actually
requires them or not.

This change is the first in a series to clean up this unused markup in
the tests. This first step is to just remove all @require@ and @diff@
comments where they seem to serve no purpose at all. These tests do not
open any of the *.tst or *.txt files that are copied into the test's
working directory from the testsuite/data directory, so they don't
"require" any of those files, and there's no need to "diff" them after
the test runs.

libstdc++-v3/ChangeLog:

	* testsuite/27_io/basic_filebuf/close/char/4879.cc: Remove
	@require@ and @diff@ comments.
	* testsuite/27_io/basic_filebuf/close/char/9964.cc: Likewise.
	* testsuite/27_io/basic_filebuf/open/char/3.cc: Likewise.
	* testsuite/27_io/basic_filebuf/open/char/9507.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sbumpc/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sbumpc/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sgetc/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sgetc/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sgetn/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sgetn/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/snextc/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sputbackc/char/1-io.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/sputbackc/char/1-out.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/sputbackc/char/2-io.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/sputbackc/char/2-out.cc:
	Likewise.
	* testsuite/27_io/basic_filebuf/sputc/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sputc/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sputn/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sputn/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sungetc/char/1-io.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sungetc/char/1-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sungetc/char/2-io.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sungetc/char/2-out.cc: Likewise.
	* testsuite/27_io/basic_filebuf/sputc/char/1-io.cc: Likewise.
	Remove unused variable.
	* testsuite/27_io/basic_filebuf/sputn/char/2-io.cc: Likewise.
	* testsuite/27_io/basic_ofstream/cons/char/1.cc: Remove
	@require@ and @diff@ comments. Remove unused variables.
	* testsuite/27_io/basic_ofstream/rdbuf/char/2832.cc: Remove
	* testsuite/27_io/ios_base/sync_with_stdio/2.cc: Likewise.

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