fix: ensure gengodep uses vendor dir if present

In a previous PR, initialization of the `srcdir` varible was removed
from `makeit/gengodep`. However, the `gengodep` script was still using
`srcdir` when checking for a `${srcdir}/vendor/modules.txt`.

sylabs@48e7fab

As a result, the vendor dir is never found, and `go` was run with
`-mod=readonly`, which would lead to downloads.

Since go 1.14 the presence of a vendor dir infers `-mod=vendor`
automatically, so we can just remove the handling here.

Fixes sylabs#461

makeit/gengodep

@@ -28,15 +28,9 @@ shift 4
 # get propagated down to go list.
 export GOPROXY
 
-if test -e "${srcdir}/vendor/modules.txt" ; then
-	mod_mode=vendor
-else
-	mod_mode=readonly
-fi
-
 template='{{ with $d := . }}{{ if not $d.Standard }}{{ range $d.GoFiles }}{{ printf "%s/%s\n" $d.Dir . }}{{ end }}{{ range $d.CgoFiles }}{{ printf "%s/%s\n" $d.Dir . }}{{ end }}{{ end }}{{ end }}'
 
-godeps=`${go} list -mod=${mod_mode} -deps -f "${template}" -tags "${gotags}" "$@" | sort -u`
+godeps=`${go} list -deps -f "${template}" -tags "${gotags}" "$@" | sort -u`
 
 for m in ${godeps}; do
     echo "$var += $m" >> ${depfile}