In share package mathml, revise test script, and general cleanup.
Revise test script mathmltest:
* get script running again
* format input and output expressions as MathML
* output file is now tmp-mathmltest-output.xml
* new minimal example.mac to serve as input for test script
* cut out stuff about Internet Explorer from test script
* remove file containing XML entity data for Internet Explorer
General cleanup:
* extract Texinfo from patch file into new, minimal Texinfo file
* remove patch file (ignoring changes to other parts of Maxima)
* update README to remove obsolete text