Remove unused variables from riscv-fbsd-tdep.c

Remove unused variables from riscv-fbsd-tdep.c

This removes a couple of unused variables from riscv-fbsd-tdep.c.
This allows a --enable-targets=all build to complete on
x86-64 Fedora 28.

gdb/ChangeLog
2018-10-08  Tom Tromey  <tom@tromey.com>

	* riscv-fbsd-tdep.c (riscv_fbsd_sigframe_init): Remove unused
	variable.
	(riscv_fbsd_init_abi): Likewise.