tools/node-modules: remove redundant top_srcdir
tools/node-modules: remove redundant top_srcdir

We already change the directory to the toplevel srcdir of the project at
the very start of the script.  We don't need to do it also on entry to
main: remove the redundant variable and `cd`.

martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit