remove lcsymtacs (github issue #666)

The lcsymtacs structure is regarded superseded as it plainly is
a shorthand for opening the following modules:
Abbrev HolKernel boolLib Tactic Tactical BasicProvers simpLib
Rewrite bossLib Thm_cont

Mechanisation of an elegant proof by Masako Takahashi

This is the first proof in "Parallel Reduction in λ-Calculus" from
Information and Computation 118 (1995).