Fix clang 15 warnings: (#4325)

Clang warned about the code removed in this patch with the warning:
```
warning: out-of-line definition of constexpr static data member is
redundant in C++17 and is deprecated [-Wdeprecated]
```

src/ripple/core/impl/Config.cpp

@@ -263,8 +263,6 @@ getEnvVar(char const* name)
     return value;
 }
 
-constexpr FeeUnit32 Config::TRANSACTION_FEE_BASE;
-
 Config::Config()
     : j_(beast::Journal::getNullSink()), ramSize_(detail::getMemorySize())
 {