aadwarf64-morello: .cfi_startproc purecap syntax is unnecessary

[nsz-arm]

https://github.com/ARM-software/abi-aa/blob/main/aadwarf64-morello/aadwarf64-morello.rst#cfi-assembler-syntax-for-pure-capability-functions

the spec recommends special syntax to mark purecap functions, but the assembler should know which functions are purecap already for interworking, so it can generate CFI based on that (which is more reliable and consistent with existing practice: the meaning of the cfi directives depend on the target arch and abi settings).

if we assume that base lp64 abi functions and purecap abi functions can be mixed on a call stack and there are no further capability abi variants then there is no need for special syntax (it can just cause trouble in asm code when the target abi setting and cfi directive are inconsistent).

[jrtc27]

Note that CHERI-MIPS and CHERI-RISC-V do not have such an addition