libstdc++: Remove bogus assertion in std::from_chars [PR105324]

I'm not sure what I was thinking when I added this assertion, maybe it
was supposed to be alignment == 1 (which is what the pmr::string actually
uses). The simplest fix is to just remove the assertion.

The assertion is no longer enabled by default on trunk, but it's still
there for the --enablke-libstdcxx-debug build, and is still wrong. The
fix is needed on the gcc-11 branch.

libstdc++-v3/ChangeLog:

	PR libstdc++/105324
	* src/c++17/floating_from_chars.cc (buffer_resource::do_allocate):
	Remove assertion.
	* testsuite/20_util/from_chars/pr105324.cc: New test.