powerpc/64s: Remove the "fast endian switch" syscall

The non-standard "fast endian switch" syscall was added in 2008[1],
but was never widely used. It was disabled by default in 2017[2], and
there's no evidence it's ever been used since.

Remove it entirely.

A normal endian switch syscall was added in 2015[3].

[1]: 745a14cc ("[POWERPC] Add fast little-endian switch system call")
[2]: 529d235a ("powerpc: Add a proper syscall for switching endianness")
[3]: 727f1361 ("powerpc: Disable the fast-endian switch syscall by default")

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240823070830.1269033-1-mpe@ellerman.id.au