proc: Remove assertion that P_WEXIT is not set in proc_rwmem()

exit1() sets P_WEXIT before waiting for holding threads to finish,
rather than after, so this assertion is racy.

Fixes: 12fb39ec3e6b ("proc: Relax proc_rwmem()'s assertion on the process hold count")
Reported by: Jenkins

(cherry picked from commit 879b0604a8940f575f03bde5457bcf13e64c1ae8)