x86-32: Remove unused but set variables

Spotted by GCC:

arch/x86/insn-selector_32.c: In function 'save_invoke_result':
arch/x86/insn-selector_32.c:608:19: warning: variable 'esp' set but not used [-Wunused-but-set-variable]
arch/x86/insn-selector_32.c:1902:25: warning: variable 'eax' set but not used [-Wunused-but-set-variable]
arch/x86/insn-selector_32.c:1927:25: warning: variable 'eax' set but not used [-Wunused-but-set-variable]

Signed-off-by: Pekka Enberg <penberg@kernel.org>