Move test for whitespace stripping out of function.

Rather than have the function call be a noop, which is misleading and a waste of
resources, this commit simply moves the conditional test up to the call site. If
g:spf13_keep_trailing_whitespace exists then we simply don't call the Strip()
function. This way Strip() can just do its job when called. I, for example, have
<leader>ws bound to this function and call it manually when needed. Hiding the
conditional inside the function broke that.