Don't depend on Flycheck

[mekeor]

Depending on third-party package Flycheck doesn't seem necessary. (Since Emacs' built-in Flymake has evolved over the last years, some users might prefer to not install Flycheck.) Here's a list of lines of code that depend on Flycheck:

lean4-mode.el:157:    ["List of errors"       flycheck-list-errors               flycheck-mode]
lean4-mode.el:167:    (flycheck-after-syntax-check-hook . lean4-info-buffer-redisplay-debounced)
lean4-mode.el:182:  (setq-local flycheck-disabled-checkers '())