Remove $wgUseTeX for checking if editing tools should be shown

The EditPage toolbar used the global $wgUseTeX to determine, if
the icon for inserting math should be shown. Now we simply check
if the class MathRenderer exits.

Change-Id: I0a0203f2d8c295bfc56694eb08c9986420c086cf
Merge "Remove $wgUseTeX for checking if editing tools should be shown"