LoongArch: gas: Fix shared builds

Formerly an include of libbfd.h was added in commit 56576f4a
("LoongArch: gas: Add support for linker relaxation."), in order to
allow calling _bfd_read_unsigned_leb128 from gas, but doing so broke
shared builds. Commit d2fddb6d fixed this reference but did not
remove the now unnecessary inclusion of libbfd.h. The gas_assert macro
expands into a conditional call to abort(), but "abort" is re-defined to
_bfd_abort in libbfd.h, so the extra include breaks any gas_assert
usage, and should be removed.

gas/ChangeLog:

	* config/tc-loongarch.c: Don't include libbfd.h.

Fixes: d2fddb6d ("LoongArch: Fix ld "undefined reference" error with --enable-shared")
Signed-off-by: WANG Xuerui <git@xen0n.name>