Revert "Enable building devtools by default in /browser/confvars.sh"

This reverts commit b12e52bb1a9fec62c15d2e9d123fed97372ac5bb.
Build devtools by default, switch configure parameter to optionally disable.
When switching Developer Tools to enabled by default, the configure option needed would be to disable them (countering the default).
Combined the previous commit for confvars with this one for easy reversal of defaults if desired in the future.