texi2any "SILENT" variable appears to do nothing

[Gavin Smith]

I found that SILENT was reported as missing in one of the check scripts
under doc/refcard:

make vcheck:

man only   1: SILENT

txivarcheck was changed to ignore this variable in texi2any on
2024-04-06 (fdc9653fa8adf13a):

    * doc/refcard/txivarcheck (read_tp): filter out the 'SILENT'
    customization variable that is not interesting to document as it does
    nothing, and the customization variables starting with XS_ that are
    not documented on purpose.

SILENT is still documented in the manual as equivalent to --silent.  However,
it does not work.  For example,

../tp/texi2any.pl --pdf --silent ../doc/info-stnd.texi

prints nothing.  But

../tp/texi2any.pl --pdf -c SILENT=1 ../doc/info-stnd.texi 

prints all the output from pdftex that you would expect.

I am going to try to remove this variable and any mention in the documentation.