PR binutils/22875: MIPS: Remove duplicate unsupported relocation processing

PR binutils/22875: MIPS: Remove duplicate unsupported relocation processing

Remove a duplicate `unsupported relocation type' message and the setting
of the `bfd_error_bad_value' error from `mips_elf32_rtype_to_howto',
added with commit f3185997ac09 ("PR 22875: Stop strip corrupting unknown
relocs"), <https://sourceware.org/ml/binutils/2018-02/msg00445.html>.
This message is already produced and the `bfd_error_bad_value' error set
by `mips_elf32_rtype_to_howto' before a NULL howto is returned, so there
is no need to repeat these actions here.

	bfd/
	* elf32-mips.c (mips_info_to_howto_rel): Remove the calls to
	`_bfd_error_handler' and to set the `bfd_error_bad_value' error.