DERBY-6075: Use modern collections in impl/sql/compile

- Remove dead code for old syntax that allowed declaring columns as
  nullable explicitly.

- Remove unneeded imports.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1456369 13f79535-47bb-0310-9956-ffa450edef68
DERBY-6075: Use modern collections in impl/sql/compile

Replace the remaining Vectors in sqlgrammar.jj with ArrayLists.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1456370 13f79535-47bb-0310-9956-ffa450edef68