powerpc/asm-offset: Remove unused items

Following PACA related items are not used anymore by ASM code:
PACA_SIZE, PACACONTEXTID, PACALOWSLICESPSIZE, PACAHIGHSLICEPSIZE,
PACA_SLB_ADDR_LIMIT, MMUPSIZEDEFSIZE, PACASLBCACHE, PACASLBCACHEPTR,
PACASTABRR, PACAVMALLOCSLLP, MMUPSIZESLLP, PACACONTEXTSLLP,
PACALPPACAPTR, LPPACA_DTLIDX and PACA_DTL_RIDX.

Following items are also not used anymore:
SIGSEGV, NMI_MASK, THREAD_DBCR0, KUAP, TI_FLAGS, TI_PREEMPT,
DCACHEL1BLOCKSPERPAGE, ICACHEL1BLOCKSIZE, ICACHEL1LOGBLOCKSIZE,
ICACHEL1BLOCKSPERPAGE, STACK_REGS_KUAP, KVM_NEED_FLUSH, KVM_FWNMI,
VCPU_DEC, VCPU_SPMC, HSTATE_XICS_PHYS, HSTATE_SAVED_XIRR and
PPC_DBELL_MSGTYPE.

Remove all of them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1c80981548dc0c4f145109cdd473022c1aad8d2b.1620223302.git.christophe.leroy@csgroup.eu