Drop the cdylib export of lightning-invoice

[TheBlueMatt]

There are ~zero functions in lightning-invoice that are materially
callable from C, so there isn't any reason to tag it as a cdylib
(and make rustc build it as such). Instead, we have C bindings now.

[codecov]

Codecov Report

Merging #968 (b786adf) into main (bfd9646) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #968      +/-   ##
==========================================
+ Coverage   90.65%   90.67%   +0.02%     
==========================================
  Files          60       60              
  Lines       30407    30407              
==========================================
+ Hits        27566    27573       +7     
+ Misses       2841     2834       -7     

Impacted Files	Coverage Δ
lightning/src/ln/channelmanager.rs	83.90% <0.00%> (+0.04%)	⬆️
lightning/src/ln/functional_tests.rs	97.22% <0.00%> (+0.09%)	⬆️

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