test/ObjectMap: Remove unused CONTROL_PREFIX variable

The `CONTROL_PREFIX` constant in `test_keyvaluedb_atomicity.cc` has
never been used since its introduction in commit 6a624b9.

Clang's warning highlights the unused variable:
```
    warning: unused variable 'CONTROL_PREFIX' [-Wunused-const-variable]
```
Remove the unnecessary constant to clean up the code and eliminate
the compiler warning.

Refs 6a624b9

Signed-off-by: Kefu Chai <[email protected]>

Author: tchaikov

src/test/ObjectMap/test_keyvaluedb_atomicity.cc

@@ -15,7 +15,6 @@
 
 using namespace std;
 
-const string CONTROL_PREFIX = "CONTROL";
 const string PRIMARY_PREFIX = "PREFIX";
 const int NUM_COPIES = 100;
 const int NUM_THREADS = 30;