Since there is no need to patch libsqlite3 for our purposes, and since
libsqlite3 ≥ 3.3.9 (which is our current requirement) is widely
available on distros, there is no reason anymore to bundle the library.
Besides removing the bundled libsqlite, and adapting the configuration
respectively, we also fix the use of the SQLITE_ENABLE_COLUMN_METADATA
compile time constant to detect whether sqlite3_column_table_name() is
available by a working feature detection (otherwise bug_42589.phpt
would fail). We also skip bug73068.phpt for libsqlite 3.11.0 to
3.14.1 which have a bug (<https://sqlite.org/src/info/ef360601>).
We also completely drop support for the obscure pdo_sqlite_external
extension (which could have been enabled on Windows only by passing
`--pdo-sqlite-external` to configure), since it is not needed anymore.
Furthermore, we remove references to the bundled libsqlite from
Makefile.gcov, CONTRIBUTING.md and README.REDIST.BINS.
