Merge keyboards with same physical layout

[crawfxrd]

These keyboards use the same physical layout (defined in include/board/keymap.h):

• 15in_102
• 18H9LHA04
• 18H9LHA05

Their only different is the default keymaps. They can be converted back to a single keyboard interface with different keymaps that boards can select. There was no good reason for me to have made new KB interfaces.

TODO:

• Determine how to handle in keyboard-configurator.