gdb/riscv: fix creating breakpoints at invalid addresses

gdb/riscv: fix creating breakpoints at invalid addresses

To allow breakpoints to be created at invalid addresses,
target_read_code is used instead of read_code.  This was fixed in
commit:

  commit c01660c625766e848195285cc20581b9ed7ecfe2
  Date:   Wed Apr 17 00:31:43 2019 +0100

      gdb/riscv: Allow breakpoints to be created at invalid addresses

Unfortunately, the call to read_code was left in by mistake.  The
result is that GDB will fail when trying to create the breakpoint,
rather than when trying to install the breakpoint (as is the case with
other targets).

This commit fixes this mistake and removes the offending call to
read_code.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_breakpoint_kind_from_pc): Remove call to
	read_code.