switch2.c: Remove use of REFCC and UNSIGNED_CHARS

Explicitly use `signed char` or `unsigned char`, rather than
```

signed char k;
char k;

signed char k;
char k;
```
This should have resulted in the same thing; however, note that
`REFCC` was never defined, and `common.h` was not included, so
the old code in fact tested `char` then `unsigned char`,
which are the same.

The only difference is that a switch using plain `char` is not
tested, but since this is the same as either `signed char` or
`unsigned char`, the lack of test coverage seems relatively safe.

test/ref/Makefile

@@ -54,7 +54,7 @@ $(WORKDIR):
 
 $(WORKDIR)/%.ref: %.c | $(WORKDIR)
 	$(if $(QUIET),echo ref/$*.host)
-	$(CC) $(CFLAGS) -DREFCC -o $(WORKDIR)/$*.host $< $(NULLERR)
+	$(CC) $(CFLAGS) -o $(WORKDIR)/$*.host $< $(NULLERR)
 	$(WORKDIR)$S$*.host > $@
 
 $(DIFF): ../bdiff.c | $(WORKDIR)