Remove highlighting KEYWORDS for MODE.
MODE should be a symbol, the major mode command name, such as `c-mode'
or nil. If nil, highlighting keywords are removed for the current buffer.
To make the removal apply to modes derived from MODE as well,
pass nil for MODE and add the call to MODE-hook. This may fail
for some derived modes if some involved major mode does not
follow the standard conventions. File a bug report if this
happens, so the major mode can be corrected.