malta: Remove ".set mips32" directive

We always build for a mips32 or higher ISA, so this ".set mips32"
directive is redundant. Once MIPSr6 support is added it will become
harmful since some instruction encodings change & this directive will
cause the older encodings to be incorrectly emitted instead of the
appropriate ones for the build.

In preparation for supporting MIPSr6, remove this redundant directive.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>