Remove unused include statement in driver.cpp (#3921)

This removes unused include
```
#include "toolchain/diagnostics/diagnostic_emitter.h"
```
from `./toolchain/driver/driver.cpp`

toolchain/driver/driver.cpp

@@ -20,7 +20,6 @@
 #include "toolchain/base/value_store.h"
 #include "toolchain/check/check.h"
 #include "toolchain/codegen/codegen.h"
-#include "toolchain/diagnostics/diagnostic_emitter.h"
 #include "toolchain/diagnostics/sorting_diagnostic_consumer.h"
 #include "toolchain/lex/lex.h"
 #include "toolchain/lower/lower.h"