malloc: Remove malloc hooks from fork handler

malloc: Remove malloc hooks from fork handler

The fork handler now runs so late that there is no risk anymore that
other fork handlers in the same thread use malloc, so it is no
longer necessary to install malloc hooks which made a subset
of malloc functionality available to the thread that called fork.

(cherry picked from commit 8a727af925be63aa6ea0f5f90e16751fd541626b)