EXPERIMENTAL: Drop customizability of input method

Fixes issue #93 "Rework Abbreviations for Input
Method" (#93).

This is an experimental commit.  It's not yet decided if we'll
implement it as such.  There's an ongoing survey if anyone uses the
customizability features of Lean4 input method.

- Delete data/abbreviations.json.
- Hard-code all quail rules into lean4-input.el directly.
- Drop all complexity of lean4-input.el.
- Drop copyright notice from agda-input.el in lean4-input.el because
  it's not based on it anymore.
- Use "Lean4" as name for input method.
- Use "L4" as abbreviation for the input method in the mode line so
  that it's different from Lean(3)-Mode.