Remove Homebrew/pip cleanup from the installation script (rebased onto dev_4_4)



This is the same as gh-1216 but rebased onto dev_4_4.

----

This cleanup step will be handled in the job directly most certainly using
``rm -rf /usr/local/*`` to decrease the maintenance burden.
The TESTING_MODE variable only stipulates whether homebrew-alt PRs should be
merged for now.