gdb/amd64: clean up unused variable

When building with clang 15, I got this,

  CXX    amd64-tdep.o
amd64-tdep.c:1410:13: error: variable 'insn' set but not used[-Werror,-Wunused-but-set-variable]
    gdb_byte *insn = insn_details->raw_insn + modrm_offset;
                ^
1 error generated.

The function that uses this variable has been removed in this commit,

commit 870f88f7
Date:   Mon Apr 18 13:16:27 2016 -0400

    remove trivialy unused variables

Fix this by removing unused variable.

Tested by rebuilding on x86_64-linux with clang 15 and gcc 12.