Internals: Remove dead code, fix or sign off coverage

Remove/fix/signoff uncontroversial code coverage holes. Also added a
couple TODOs that should be investigated at some point

src/V3Error.cpp

@@ -104,7 +104,7 @@ void V3ErrorGuarded::vlAbortOrExit() VL_REQUIRES(m_mutex) {
         VL_GCOV_DUMP();
         // Exit without triggering any global destructors. Used to prevent
         // detached V3ThreadPool jobs accessing destroyed static objects.
-        ::_exit(1);
+        ::_exit(1);  // LCOV_EXCL_LINE
     }
     v3Global.vlExit(1);
 #else
@@ -375,7 +375,7 @@ void V3Error::vlAbort() {
     v3Global.shutdown();
 #endif
     VL_GCOV_DUMP();
-    std::abort();
+    std::abort();  // LCOV_EXCL_LINE
 }
 std::ostringstream& V3Error::v3errorPrep(V3ErrorCode code) VL_ACQUIRE(s().m_mutex) {
     V3Error::s().m_mutex.lock();