Remove compatibility code for Sanitizer::validateEmail

Sanitizer::validateEmail was added in 1.18, mw.org says min version is
1.19+, so this compat code is no longer needed.

Change-Id: I1ba1a8bd645c7e1c6cabe14f5efebfb8128ebf19
    Update RELEASE-NOTES.md

    [skip ci]
Merge pull request #390 from nischayn22/rel_notes

    Update RELEASE-NOTES.md
Merge "Remove compatibility code for Sanitizer::validateEmail"