Remove UNUSED_PINS

[tzarc]

Description

As it says on the tin.

Builds that had something to do with non-standard (and actually unused) UNUSED_PINS all have matching SHA1's before and after this changeset:

                                ferris/0_1:default - 292a7dafd43c030342f045b5675c2b91dcbc8b48
                          ferris/0_2/bling:default - 1f0078dc26868f7d51e5dacfef29f541f39d78ea
                        ferris/0_2/compact:default - fd55869e43302e57600dcbe147342f8c17a3f000
                           ferris/0_2/high:default - 3496d25e21345fe1a1ac3f8f19ff4a67655737d4
                           ferris/0_2/mini:default - fa86aeab5b523a4bb96383821733e0a1a14abe1d
                                ferris/0_2:default - cacda21ac0a306fca2a4b5d9f2245d7aca3b06b8
                              ferris/sweep:default - e2c66a5708c426795b2ed4b594fbbf2366994779
                            ploopyco/mouse:default - 9cfad73c6d8c9440bdc8e8390411354e324d8c38
                   ploopyco/trackball/rev1:default - 8320f437a1514b8aacb0a9ebf4bca241228e27d0
               ploopyco/trackball/rev1_005:default - f90269b5bf806b34bc7770579d79d30e029ae8b8
          ploopyco/trackball_mini/rev1_001:default - edbe0c9f16f11ad4dcab87dffc4388271c490124
          ploopyco/trackball_mini/rev1_002:default - edbe0c9f16f11ad4dcab87dffc4388271c490124
          ploopyco/trackball_nano/rev1_001:default - ac5e2a48f4571b4fc472ded59e08794f8e4abfb4

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).