Removed some temporary case-check error messages.

- Control flow statements (break, case, continue, default, do, for, if,
  switch, whilei, else) and sub-function declarations (function) will no
  longer show a custom error message when they are incorrectly
  capitalized.
- Note: This does not mean that they are no longer case-sensitive. This
  only means that no special message will be shown in case such an error
  is detected. The script engine will throw a normal error, as it would
  with any other kind of typo, rather than treating this as a special
  case. Enough time was given to update third party scripts, and it is
  now time to clean up our parsing code and get rid of these special
  warnings.
- Variables, labels and commands will still show a warning for the time
  being, since the side-effects of silently ignoring it could be quite
  dangerous. An alternative approach to the case-check will be presented
  in future. For now, those who want to disable it (and save a bit on
  execution time and memory use), can simply comment out the #define
  ENABLE_CASE_CHECK line in script.h, as usual.

Signed-off-by: Haru <haru@dotalux.com>