gdb/riscv: Remove partial target description support

gdb/riscv: Remove partial target description support

Some parts of the target description support were committed with the
initial riscv patch.  As target descriptions are not currently supported
on riscv this commit removes the two pieces for code that relate to
target description support.

It is expected that target description support will be added in the
future, at which point this, or similar code will be added back.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_register_name): Remove target description
	support.
	(riscv_gdbarch_init): Remove target description check.