Revert "Export utf8_countTrailBytes for sqlite's benefit"

This doesn't work because utf8_countTrailBytes is only decorated with
U_EXPORT, but not `extern`. This causes a C2734 error.

This reverts commit 113fc09026285b9eb69eee15a3648e5027563d45.
Revert "Remove special-casing of sqlite3"

Turns out it's still needed.

This reverts commit 36ff5be7bbfd14678ba9c5105abe4b0f83419e6f.