Fix compiler error by deleting GetContext default ctor (#5685)

Summary:
When updating compiler version for MyRocks I'm seeing this error with rocksdb:

```
ome/yzha/mysql/mysql-fork2/rocksdb/table/get_context.h:91:3: error: explicitly defaulted default constructor is implicitly deleted
      [-Werror,-Wdefaulted-function-deleted]
  GetContext() = default;
  ^
/home/yzha/mysql/mysql-fork2/rocksdb/table/get_context.h:166:18: note: default constructor of 'GetContext' is implicitly deleted because field
      'tracing_get_id_' of const-qualified type 'const uint64_t' (aka 'const unsigned long') would not be initialized
  const uint64_t tracing_get_id_;
                 ^
```

The error itself is rather self explanatory and makes sense.

Given that no one seems to be using the default ctor (they shouldn't, anyway), I'm deleting it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5685

Differential Revision: D16747712

Pulled By: yizhang82

fbshipit-source-id: 95c0acb958a1ed41154c0047d2e6fce7644de53f

table/get_context.h

@@ -95,7 +95,7 @@ class GetContext {
              ReadCallback* callback = nullptr, bool* is_blob_index = nullptr,
              uint64_t tracing_get_id = 0);
 
-  GetContext() = default;
+  GetContext() = delete;
 
   // This can be called to indicate that a key may be present, but cannot be
   // confirmed due to IO not allowed