[ VM / Service ] Remove bool return value for service RPC handlers

Returning 'false' from a RPC handler in service.cc results in the VM
shutting down due to an UNIMPLEMENTED error. Since we always return
'true', even in the case of RPC errors, and the TODO to handle 'false'
returns is over 6 years old, RPC handlers should just have a void
return.

Example: invoking getSourceReport on an AOT VM should just return a RPC
error, but 'false' was being returned from GetSourceReport causing the
VM to shutdown.

Fixes flutter/devtools#2955.

TEST=Existing service tests

Change-Id: I344c2f4cf849d27f08f59e8c61fb4c17c32719d0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/197241
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>

Author: bkonyi