[Examples] Delete the original deprecated GreetingService examples (#483)

### Motivation

Depends on #472
landing first.

Removes the original, now deprecated, GreetingService and
GreetingServiceClient. We kept them here until close to 1.0 as pre-1.0
tutorials point to them.

### Modifications

Deleted the examples, they're obsolete now and have been replaced by
better examples.

### Result

No more deprecated examples.

### Test Plan

N/A

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Co-authored-by: Si Beaumont <[email protected]>

Author: czechboy0