guardianvideos: remove hardcoded api-key

We should use the one from GrlConfig instead.

Note that we got support to per source GrlConfig with grilo@6ebd283d (since grilo-0.3.7) so an user could have a ini file with

[grl-lua-factory grl-guardianvideos-lua]
api-key=my-api-key-goes-here

And have GRL_CONFIG_PATH point to that file (see grilo!71 (merged)) - but that should be available only in the next Grilo release (grilo 0.3.14)