fix el-get-remove-autoloads for emacs != 24.3

Instead of tricking `autoload-find-destination` to do what we want, pull
out the relevant bits of code that do removal.

Using `autoload-find-destination` works in emacs 24.3 apparently because
of a bug: file-attributes returns a 4 integer list for time, but the
autoload function expects the old 2 integer format.  In 24.2 where
`file-attributes` still returns 2 integers, the autoload section is
considered up to date and is not removed.

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This fixes #446 for emacs older than 24.3. I've tested (`./run-test.sh 446`) with 23.4, 24.2, and 24.3.