remove DOCS from default target and druntime.zip

- These are the raw b&w ddoc html files that nobody uses.

- The distributed druntime docs are generated
  by dlang.org (through the doc target).
move -version=CoreDdoc to DDOCFLAGS

- DOCFMT is typically overriden when calling
  the makefile.
Merge pull request #667 from MartinNowak/rmDOCS

[install] remove DOCS from default target and druntime.zip