Don't quote assert expressions in DairikiDiff

Don't quote assert expressions in DairikiDiff

Per HHVM issue 5128, it is not possible to use '$this' in string-literal
assert() expressions. We can either wait for this to be fixed (unlikely to
happen soon, since it involves deep interpreter internals), comment out or
remove the asserts, or simply unquote them, so that they are actual expressions
rather than strings. The downside to this is that assertions will always be
evaluated (but so what, they are extremely cheap), and that when an assertion
fail the error message will simply read 'assert(): Assertion failed in
/path/to/file on line XXX' as opposed to including the expression in the
output. Fair trade, IMO.

See: https://github.com/facebook/hhvm/issues/5128

Bug: T124163
Change-Id: Ib458b1b0c28f8d38e9df427196ae79814f6dc0c2