Revert "Make the idle timer not be buffer local"

[joaotavora]

This reverts commit cc8da01.

Since this commit, interaction with flymake-mode in .el buffers (and probably some other modes) is broken.

To reproduce:

emacs -Q -l aggressive-indent.el -f global-aggressive-indent-mode aggressive-indent.el -f flymake.mode ;; use the aggressive-indent.el file itself as a test fixture
C-s s t r i n g p ;; search forward to an appropriate sexp (should land you around line 176)
C-M-u ;; to go to the beginning of this sexp
C-M ;; newline

The form is not indented as desired.

If I revert the commit, everything is OK.

[joaotavora]

By the way, this has no relation to #119, it happens with and without that change.