gh-145685: Remove _PySeqLock that is not used anymore (gh-155243)

[maurycy]

As per #145685 (comment):

maurycy@gimel cpython (main 9ccd5bb) % rg _PySeqLock
Objects/typeobject.c
11:#include "pycore_lock.h"          // _PySeqLock_*

Include/internal/pycore_lock.h
221:} _PySeqLock;
224:PyAPI_FUNC(void) _PySeqLock_LockWrite(_PySeqLock *seqlock);
227:PyAPI_FUNC(void) _PySeqLock_UnlockWrite(_PySeqLock *seqlock);
231:PyAPI_FUNC(void) _PySeqLock_AbandonWrite(_PySeqLock *seqlock);
234:PyAPI_FUNC(uint32_t) _PySeqLock_BeginRead(_PySeqLock *seqlock);
238:PyAPI_FUNC(int) _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous);
242:PyAPI_FUNC(int) _PySeqLock_AfterFork(_PySeqLock *seqlock);

Python/lock.c
579:void _PySeqLock_LockWrite(_PySeqLock *seqlock)
600:void _PySeqLock_AbandonWrite(_PySeqLock *seqlock)
607:void _PySeqLock_UnlockWrite(_PySeqLock *seqlock)
614:uint32_t _PySeqLock_BeginRead(_PySeqLock *seqlock)
625:int _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous)
640:int _PySeqLock_AfterFork(_PySeqLock *seqlock)

Hardly newsworthy?

• Issue: Unexpected scaling bottleneck in _PyType_Lookup / _PyType_LookupStackRefAndVersion #145685