Style: Remove unnecessary semicolons from core

Semicolons are not necessary after function definitions or control flow
blocks, and having some code use them makes things inconsistent (and
occasionally can mess up `clang-format`'s formatting).

Removing them is tedious work though, I had to do this manually (regex
+ manual review) as I couldn't find a tool for that. All other code
folders would need to get the same treatment.

core/os/thread.cpp

@@ -63,4 +63,4 @@ Error Thread::set_name(const String &p_name) {
 	}
 
 	return ERR_UNAVAILABLE;
-};
+}