match-case: eliminate useless initial flag test.

* share/txr/stdlib/match.tl (match-case): The first case
should not test the flag variable, the variable is false.
This compiles to a useless if instruction and unreachable code.
I tried writing a peephole rule against that instruction
sequence in an experimental peephole optimizer, but across the
entire code, it only matched in code in compiler.tl arising
out of match-case, so it is better to squash this at the
source. I won't commit the peephole optimizer until it comes
up with something that isn't better fixed elsewhere.

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