Dropping deprecated database API function db_query()

Following recent commits db_query() is no longer used in MantisBT.
- 3be86ce37bb469b211663da03f61ac2b28500d91,
- 7d76827617e97d7356e7eb4f2f66736e4c4d41b0,
- cf596c27b2a2e2aac67749b546cece9c23b4bbb1 and
- 0db530a855f92752aaa4bcf355df3ecba9af8226

Considering that the function is marked as deprecated since 1.2.0a2
(April 2008), it is time to effectively remove it.