efi: loongarch: Drop exports of unused string routines

Drop the __efistub_ prefixed exports of various routines that the EFI
stub on LoongArch does not even use.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/loongarch/kernel/image-vars.h

@@ -7,12 +7,7 @@
 
 #ifdef CONFIG_EFI_STUB
 
-__efistub_memchr		= memchr;
-__efistub_strcat		= strcat;
 __efistub_strcmp		= strcmp;
-__efistub_strncat		= strncat;
-__efistub_strnstr		= strnstr;
-__efistub_strrchr		= strrchr;
 __efistub_kernel_entry		= kernel_entry;
 __efistub_kernel_asize		= kernel_asize;
 __efistub_kernel_fsize		= kernel_fsize;