Ignore value of EMACS if INSIDE_EMACS is set

Ignore value of EMACS, regardless of its value, if INSIDE_EMACS is set.

According to the Emacs manual, EMACS should be set to "t" if
INSIDE_EMACS is set:

  36.2 Interactive Subshell

  Emacs sets the environment variable `INSIDE_EMACS' in the subshell to
  `VERSION,comint', where VERSION is the Emacs version (e.g., `24.1').
  Programs can check this variable to determine whether they are running
  inside an Emacs subshell. (It also sets the `EMACS' environment
  variable to `t', if that environment variable is not already defined.
  However, this environment variable is deprecated; programs that use it
  should switch to using `INSIDE_EMACS' instead.)

However, EMACS is set to a string containing the Emacs version in
24.3.50.1:

$ env | grep EMACS
EMACS=24.3.1 (term:0.96)
INSIDE_EMACS=24.3.1,term:0.96

Instead of relying on a known value of EMACS when INSIDE_EMACS is set,
just ignore the value of EMACS.