x86/mm: Remove duplicated word in warning message

Commit bbeb69ce ("x86/mm: Remove CONFIG_HIGHMEM64G support") introduces
a new warning message MSG_HIGHMEM_TRIMMED, which accidentally introduces a
duplicated 'for for' in the warning message.

Remove this duplicated word.

This was noticed while reviewing for references to obsolete kernel build
config options.

Fixes: bbeb69ce ("x86/mm: Remove CONFIG_HIGHMEM64G support")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kernel-janitors@vger.kernel.org
Link: https://lore.kernel.org/r/20250516090810.556623-1-lukas.bulwahn@redhat.com