refactor(aarch64): remove `FEAT_BTI` architecture check

BTI instructions are a part of the NOP space in earlier architecture
versions, so it's not inherently incorrect to enable BTI code
or instructions even if the target architecture does not support them.

This change reduces our reliance on architecture versions when checking
for features.

Change-Id: I79f884eec3d65978c61e72e4268021040fd6c96e
Signed-off-by: Chris Kay <chris.kay@arm.com>

include/arch/aarch64/asm_macros.S

@@ -10,10 +10,6 @@
 #include <common/asm_macros_common.S>
 #include <lib/spinlock.h>
 
-#if ENABLE_BTI && !ARM_ARCH_AT_LEAST(8, 5)
-#error Branch Target Identification requires ARM_ARCH_MINOR >= 5
-#endif
-
 /*
  * TLBI instruction with type specifier that implements the workaround for
  * errata 813419 of Cortex-A57 or errata 1286807 of Cortex-A76.