revert: removing read_committed argument from outer_atomic function (#28161)

In the PR https://github.com/edx/edx-platform/pull/10659 the outer_atomic decorator/context manager was created to prevent nested atomic blocks. This method received a boolean parameter read_committed to enforce read-committed MySQL isolation level. From Django 2, the default isolation level Django sets is read-committed, so the aforementioned parameter for outer_atomic can be removed