sqldiff: Remove MySQL workaround, causes more issues then it solves.

Remove MySQL workaround for "weird bug". The original bug cannot be
tracked down nor reproduced. However, the workaround code produces
real bugs when detecting varchar display size. Removing the workaround
produces better, more accurate results for MySQL in many real test
scenarios.

Fixes #532
Merge pull request #543 from jdufresne/master

sqldiff: Remove MySQL workaround, causes more issues then it solves.