Remove Homebrew/pip cleanup from the installation script

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.
Merge pull request #1218 from sbesson/rebased/dev_4_4/homebrew_cleanup

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