DOC: coobook fix math rendering

Don't overwrite math role:
https://github.com/sphinx-doc/sphinx/issues/2927#event-773707455

Documentation/Cookbook/rst/recipes/bandmathx.rst

 .. highlight:: c++
 
-.. role:: math(raw)
-   :format: html latex
-..
-
 BandMathImageFilterX (based on muParserX)
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