Remove hidapi_exit() entirely

It was not supposed to be used explicitly in the first place (see
comments on the PR this commit is a part of).  The HIDAPI library will
still be cleanly finalized before the program terminates, through
another – automatic – mechanism.

Additionally, the only currently known caller is using it as a
workaround for another issue (trezor#114), and that use case would not be
supported by the no-op compatibility shim anyway.

Related: trezor#114 ("Cannot reenumarate devices in same python process")
Related: trezor#128 ("Fatal Python error: Segmentation fault on 0.11.0.post2")

hid.pyx

@@ -66,14 +66,6 @@ def enumerate(int vendor_id=0, int product_id=0):
     hid_free_enumeration(info)
     return res
 
-def hidapi_exit():
-    """No-op.
-
-    Turned into a no-op to prevent memory safety bugs (see issue #128).
-
-    HIDAPI will be finalized automatically, when appropriate.
-    """
-
 cdef class device:
     """Device class.