Revert "Update javadoc for clarification"

This reverts commit c3ce410e54f267dc209c140a3a4c06588c47fb31.
Revert "Push TigerVariable instead of VariableSymbolTableEntry"

This reverts commit f2083fddc1b5afb24f62c353517d4fddf4ba1582.
Revert "Get rid of all variable idExists() checks"

This reverts commit 4d3e01881b47810e6cdc2370488802e89f4db9c8.