Remove combohandler feature gate for 8.0 release

[danobi]

I recently noticed combohandler was marking all of its generated documents as Cache-Control: Public regardless of what the requested documents contained in Cache-Control. That's a bug. I've feature-gated the fix using a #define FEAT_GATE_8_0 in commit 9d33ca2. It should be undef'd for 8.0 since it's a breaking change.

[bryancall]

This define and ifdef should be remove for the 9.0.0 release.

[zwoop]

Blocks #5457