[test] Remove ineffective re-load guard in stressInterpreter:

even when interpreter, the JIT will take the stressReflection symbol
from the binary, where `#if !defined(__CLING__)` does not protect from
re-inclusion (and thus unloading) of stressInterpreter.cxx in the
interpreter. Make that more obvious by removing the ineffective
do-not-reinclude guards.