Also simplify updaterst target in doc/Makefile
Partially revert commit b4feb85f11 : re-add
doc/source/lib/galaxy.webapps.galaxy.api.rst and
doc/source/lib/galaxy.webapps.tool_shed.api.rst and include them in
doc/source/api/api.rst and doc/source/api/ts_api.rst respectively to
simplify doc maintenance.