databases/mantis: Revert fake update from r475644

The checksums and sizes from 2.9.0 and 2.15.0 are identical because
GH_TAGNAME was not updated as well, so the update to 2.15.0 never
actually happened.

PR: 229880
Pointy hat: joneum
MFH: 2018Q3
Security: 0822a4cf-9318-11e8-8d88-00e04c1ea73d