Remove code duplication found in gravit.sh gravity_ParseFileIntoDomains()

[yubiuser]

What does this PR aim to accomplish?:

With the transition of parseList() from gravity.sh to FTL realm some code duplication occurred. Duplicated code is from gravity_ParseFileIntoDomains()

https://github.com/pi-hole/pi-hole/blob/50d67dce73e122d98026ab2a4b30255efd9b05fc/gravity.sh#L639-L667

No all input modifications were duplicated into FTL so we could not remove gravity_ParseFileIntoDomains completely.

How does this PR accomplish the above?:

Removes the duplicates

P.S. It would also be possible to move the whole gravity_ParseFileIntoDomains also into FTL's realm, however this would require more modifications (in core and FTL

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