fontbe::features contains two impl of resolve_variable_metric; should unify them

[Cosimo Lupo]

as noted in #407 and #779 (comment)

fontc/fontbe/src/features.rs

Line 148 in fd830f0

 pub(crate) fn resolve_variable_metric<'a>( 

and

fontc/fontbe/src/features.rs

Line 320 in fd830f0

 fn resolve_variable_metric( 

the two are very similar to one another, one is used to generate deltas for handwritten variable FEA, the other for autogenerated kern/mark features