variable_watch: Remove leftover debugging code (#14187)

When a watch does not specify a command to call then variable_watch
prints out a message to stderr.  Remove code after that which collects
all variable values to construct a message that is never printed.
Otherwise such code causes a READ_ACCESS watch to trigger on all
variables in the currents scope.

Reported-by: Yichao Yu <yyc1992@gmail.com>
variable_watch: Print accesses as "CMake Debug Log" messages

Use makefile->IssueMessage() to print the unprocessed watch message in a
format consistent with other CMake messages and with a more complete
call stack for the access.
Merge topic 'fix-variable_watch-cases' into next

647745b variable_watch: Print accesses as "CMake Debug Log" messages
515f9af variable_watch: Remove leftover debugging code (#14187)