Remove incorrect statements about GemFire Java Reflection-based (PDX) Serialization in the SDG Reference Guide.

References to performance regarding Java Reflection in the Spring Data GemFire's Reference Guide, and in particular the section, Function Execution with PDX, are incorrect and need to be revised.

Specifically, the guide states...

Of course, it is recommend to use a "custom" `PdxSerializer` where possible given the performance implications of using Java's Reflection functionality.

This line will be revised to say...

Of course, it is recommend to use a "custom" `PdxSerializer` where possible to maintain finer grained control over your serialization strategy.