staging: xgifb: Drop some more useless defines

SetLCDStdMode is not supported as of 34c13ee2 ("staging: xgifb: drop
code for legacy VGA modes").

DisableLCD24bpp defined to zero was never realy useful and went away with
aa56b279 ("staging: xgifb: vb_table: delete XGI21_LCDCapList").

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>