Remove stale -DNO_PROTOTYPES bits from gdb testsuite

The gdb.base/call-sc.exp, gdb.base/structs.exp and
gdb.base/structs2.exp testcases still try compiling the sources with
-DNO_PROTOTYPES, but the corresponding sources don't have any #ifdef
NO_PROTOTYPES any longer.  Those were removed throughout years ago.

OTOH, gdb.base/ovlymgr.h does check for NO_PROTOTYPES, but no .exp
file compiles it with -DNO_PROTOTYPES.

gdb.base/reread.exp and gdb.base/varargs.exp set a 'prototypes'
global, which is a stale bit left behind when the "try-compiling
without and then with -DNO_PROTOTYPES" logic was around.

gdb/testsuite/ChangeLog:

	* gdb.base/call-sc.exp (start_scalars_test): Use
	prepare_for_testing and don't try compiling with -DNO_PROTOTYPES.
	* gdb.base/overlays.c: Remove references to PARAMS.
	* gdb.base/ovlymgr.h (PARAMS): Delete, and remove all references.
	* gdb.base/reread.exp: Don't set 'prototypes' global.
	* gdb.base/structs.exp (start_structs_test): Use
	prepare_for_testing and don't try compiling with -DNO_PROTOTYPES.
	* gdb.base/structs2.exp: Don't set 'prototypes' global.  Use
	prepare_for_testing and don't try compiling with -DNO_PROTOTYPES.
	Don't issue "set width 0".  Remove gdb_stop_suppressing_tests
	call.
	* gdb.base/varargs.exp: Don't set 'prototypes' global.