x86/speculation: Remove the extra #ifdef around CALL_NOSPEC

Commit:

  010c4a46 ("x86/speculation: Simplify and make CALL_NOSPEC consistent")

added an #ifdef CONFIG_MITIGATION_RETPOLINE around the CALL_NOSPEC definition.
This is not required as this code is already under a larger #ifdef.

Remove the extra #ifdef, no functional change.

vmlinux size remains same before and after this change:

 CONFIG_MITIGATION_RETPOLINE=y:
      text       data        bss         dec        hex    filename
  25434752    7342290    2301212    35078254    217406e    vmlinux.before
  25434752    7342290    2301212    35078254    217406e    vmlinux.after

 # CONFIG_MITIGATION_RETPOLINE is not set:
      text       data        bss         dec        hex    filename
  22943094    6214994    1550152    30708240    1d49210    vmlinux.before
  22943094    6214994    1550152    30708240    1d49210    vmlinux.after

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20250320-call-nospec-extra-ifdef-v1-1-d9b084d24820@linux.intel.com