RISC-V: Remove a loop in the ISA parser

Since commit e601909a3287bf541c6a7d82214bb387d2c76d82 ("RISC-V: Support
to parse the multi-letter prefix in the architecture string.") changed
so that all prefixed extensions are parsed in single
riscv_parse_prefixed_ext call, a "while" loop on riscv_parse_subset
is no longer required.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_subset): Remove unnecessary loop.