Delete redundant snippets; fix crash by removing .yas-parents files.

The redundant snippets are already defined in the yasnippet module,
which can now merge my personal snippets with the pre-packaged ones.

These .yas-parents files, left over from an earlier incarnation of my
snippets directory, are Considered Harmful now. It turns out that
yasnippet is (now?) smart and travels up the emacs major-mode
hierarchy to figure out which snippet modes to activate. (Emacs
`define-derived-mode` lets you define parents for new modes, which
defines that hierarchy.) When you specify the hierarchy *again* via
.yas-parents files, yas--all-parents seems to acquire cyclic
dependencies and go off into an infinite loop that requires me to kill
emacs.
Add a testing macro for Python.