Documentation/x86: Zap the subsection letters

The subsections already have numbering - no need for the letters too.

Zap the latter.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250409111435.GEZ_ZWmz3_lkP8S9Lb@fat_crate.local