Fix: remove dead code from filter interpreter

Fix: remove dead code from filter interpreter

This has been detected in the lttng-modules port of the filter
interpreter by Coverity. The intent of the code in UST is similar, and
we can find the same dead code, although Coverity may not see it as dead
code because it cannot prove that the string is not modified between the
two uses. Since we know it is not modified, remove the dead code.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>