Replace ExtractThumbParameters hook with parseParamString method

The existing parseParamString function was totally broken.
The ExtractThumbParameters is deprecated, and there are some
places in MediaWiki that will only use parseParamString, and
don't use the ExtractThumbParameters hook. Thus this commit
adapts the code in onExtractThumbParameters to be in parseParamString
and removes the use of the deprecated hook.

Change-Id: I920cf01b0424257d6acf01b08c270c4e0169cca5
Merge "Replace ExtractThumbParameters hook with parseParamString method"