build: Remove unnecessary executables from gitian release

This removes the following executables from the binary gitian release:

- test_bitcoin-qt[.exe]
- bench_bitcoin[.exe]

@jonasschnelli and me discussed this on IRC a few days ago - unlike the
normal `bitcoin_tests` which is useful to see if it is safe to run
bitcoin on a certain OS/environment combination, there is no good reason
to include these. Better to leave them out to reduce the download
size.

Sizes from the 0.12 release:
```
2.4M bitcoin-0.12.0/bin/bench_bitcoin.exe
 22M bitcoin-0.12.0/bin/test_bitcoin-qt.exe
```

0 files changed, 0 insertions, 0 deletions