Repurpose Release_Developer BUILDTYPE and remove SK_DEVELOPER.

Repurpose Release_Developer BUILDTYPE and remove SK_DEVELOPER.

The Release_Developer build type is now used to build Skia with
release level optimizations but still enable SK_DEBUG. This in turn
means that SkASSERTS and SkRTConf are available in this mode.

Further we can then remove SK_DEVELOPER as a define as it is true
iff SK_DEBUG is true.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1931903002
DOCS_PREVIEW= https://skia.org/?cl=1931903002

Review-Url: https://codereview.chromium.org/1931903002