MIPS/LD/testsuite: Fix JALR relaxation test failure with IRIX 6

MIPS/LD/testsuite: Fix JALR relaxation test failure with IRIX 6

The `mips-sgi-irix6' target only supports IRIX linker emulations, but
most JALR relaxation tests request the relevant traditional emulation
instead, causing a link failure:

./ld-new: unrecognised emulation mode: elf32btsmipn32
Supported emulations: elf32bmipn32 elf32bsmip elf64bmip

This is clearly an omission from the conversion to use the per-ABI
framework made with commit 78da84f99405 ("MIPS/LD/testsuite: Correct
mips-elf.exp test ABI/emul/endian arrangement").  These tests are also
endianness agnostic, which was missed in the conversion as well.

Remove the unnecessary explicit ABI and endianness options then and rely
on the per-ABI framework to get things right, removing this regression:

mips-sgi-irix6  -FAIL: MIPS relax-jalr-shared n32

	ld/
	* testsuite/ld-mips-elf/relax-jalr-n32-shared.d: Remove flags
	related to ABI and endianness selection from the `as' and `ld'
	tags.
	* testsuite/ld-mips-elf/relax-jalr-n64.d: Likewise.
	* testsuite/ld-mips-elf/relax-jalr-n64-shared.d: Likewise.
	* testsuite/ld-mips-elf/mips-elf.exp: Remove `as' and `ld' tag
	additions from the invocation of JALR relaxation tests.