bfd, sparc: avoid duplicated error messages on invalid relocations.

bfd, sparc: avoid duplicated error messages on invalid relocations.

This patch avoids a duplicated error message when an invalid
relocation number is read from an object file in sparc-* ELF targets:

$ strip -g test.o
strip: test.o: unsupported relocation type 0xd7
strip: test.o: unsupported relocation type 0xd7
strip: test.o: bad value

Tested in x86_64-linux-gnu, sparc64-linux-gnu and sparc-linux-gnu
targets.

bfd/ChangeLog:

2018-09-04  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* elfxx-sparc.c (_bfd_sparc_elf_info_to_howto): Do not issue an
	error when an invalid relocation is passed;  this is already done
	by `_bfd_sparc_elf_info_to_howto_ptr'.