PR# 18031 Unneeded "if" in {EIFFEL_ENV}.check_environment_variable

In routine {EIFFEL_ENV}.check_environment_variable, we have the following
fragment near the end of the routine:

   if is_valid_environment then
      -- Some code I have omitted
   end
ensure
   is_valid_environment: is_valid_environment
end

This is confusing.

The "if" statement is not needed.  If `is_valid_environment' is not true
at that point, then the postcondition will be violated.  The "if" should
probably be replaced by:

   check
      environment_valid: is_valid_environment
   end

The above check assertion is guaranteed to be true because 
`on_check_environment_failure' calls `die'.