PR 30022, concurrent builds can fail

So let's not copy .libs/libbfd.a to libbfd.a now that nothing in the
binutils-gdb source tries to link against it.

	PR 30022
	* Makefile.am (noinst_LIBRARIES, libbfd_a_SOURCES, stamp-lib),
	(libbfd.a): Delete rules.
	(CLEANFILES): Adjust to suit.