[processor/transform] Remove functions option from config

[TylerHelmuth]

Description: The transformprocessor had a setting in configuration called functions that was "no-op". Setting this field would actually break the transform processor because the TQL would have no working functions. The long term intention was that users would be able to register their own functions to be usable with the TQL, but there is no implementation to do that currently.

This PR removes the unused config option, which will work towards providing a declarative config syntax.

Link to tracking Issue: Related to #11852

Testing: Ran unit tests

Documentation: This field is not currently documented. Since the processor is alpha, we can make breaking config changes without considering backwards compatibility.

[TylerHelmuth]

/cc @kentquirk

[TylerHelmuth]

Is this a breaking change?

Technically yes. But since this component is alpha we can still make config changes in breaking ways. Also, the config option, when used, breaks the processor.

[TylerHelmuth]

@mx-psi Actually the mapstructure:"-" means it is ignored right? If that's the case the setting isn't settable via config and is only used to access functions, which is unnecessary.

[mx-psi]

@mx-psi Actually the mapstructure:"-" means it is ignored right? If that's the case the setting isn't settable via config and is only used to access functions, which is unnecessary.

Right, and it's a private field, so probably even if it had a tag you wouldn't be able to set it via mapstructure, and it does not affect the Go API 👍 You have convinced me :)