template: fix sphinx targets

Remove `usedevelop=true` options; these are not needed and can generate
a build error on some swh packages.

copier-template/tox.ini.jinja

@@ -52,7 +52,6 @@ commands =
 # breaking doc build
 [testenv:sphinx]
 allowlist_externals = make
-usedevelop = true
 extras =
   testing
 deps =
@@ -69,7 +68,6 @@ commands =
 # of swh-docs package
 [testenv:sphinx-dev]
 allowlist_externals = make
-usedevelop = true
 extras =
   testing
 deps =