Remove the translator variable tr_semicompile
The only effect of tr_semicompile:true has been wrapping most single
top-level forms in a PROGN in compfile output. This would prevent
these forms from being compiled by the lisp compiler in older dialects
like Maclisp and Franz lisp. Only subforms of a top-level PROGN form
like (PROGN 'COMPILE ...) would be compiled in these dialects.
This variable has not done anything useful in a long time, so let's
just remove it.
This was discussed on the mailing list circa 2020-02-12 in the thread
"Removing tr_semicompile".