Fix an issue where GtkIMHtmlScalables were not properly cleaned up which lead to a crash

Fix an issue where `GtkIMHtmlScalables` were not properly cleaned up which lead to a crash

Testing Done:
Followed the reproduction steps from https://issues.imfreedom.org/issue/PIDGIN-17446, found the issue with valgrind and merged the delete functions so that they actually make sense and I was unable to reproduce the issue again.

Bugs closed: PIDGIN-17446

Reviewed at https://reviews.imfreedom.org/r/514/