remove unncessary Current_thread restore in caml_thread_enter_blocking_section

[sadiqj]

Identified by @gadmm in #10966 (comment) it seems we're doing an unnecessary restore of Current_thread in Systhreads' caml_thread_enter_blocking_section.

4.14's caml_thread_enter_blocking_section for reference: https://github.com/ocaml/ocaml/blob/4.14/otherlibs/systhreads/st_stubs.c#L230

cc @Engil

[abbysmal]

This change looks good to me,
As mentioned in #10966, I think this is a leftover from the early reimplementation of systhread on Multicore.
I cannot trace it to anything meaningful, and considering caml_thread_enter_blocking_section can only be called from a thread already holding the lock (meaning, Current_thread should be set to the executing thread.), I think this change is fine.