Bug#1056811: Proposed RM: libgpuarray - abandoned, RC-buggy

[Rebecca N. Palmer]

libgpuarray has been de facto abandoned upstream since 2018, and is 
currently unused in Debian.  (Its original user was theano, which was 
removed in #1042574.)

It currently has what may or may not be a wrong-answer bug (#1031414), 
and two broken-by-transition bugs.  (The latter two _might_ be easy to 
fix - I haven't tried because I've already mostly given up on it.)

Hence, I intend to request removal.