Remove branch in v8 submodule

I suspect this is the cause of:

Submodule 'depot_tools' (https://chromium.googlesource.com/chromium/tools/depot_tools.git) registered for path 'depot_tools'
Submodule 'v8' (https://github.com/v8/v8.git) registered for path 'v8'
Cloning into '/go/src/github.com/ry/v8worker2/depot_tools'...
Cloning into '/go/src/github.com/ry/v8worker2/v8'...
From https://chromium.googlesource.com/chromium/tools/depot_tools
 * branch              f16fdf3165b8b86d10386d18a6b6075169c10e15 -> FETCH_HEAD
error: no such remote ref fe12316ec4b4a101923e395791ca55442e62f4cc
Fetched in submodule path 'v8', but it did not contain fe12316ec4b4a101923e395791ca55442e62f4cc. Direct fetching of that commit failed.
package github.com/ry/v8worker2: exit status 1

Fixes: ry#9

.gitmodules

@@ -1,7 +1,6 @@
 [submodule "v8"]
 	path = v8
 	url = https://github.com/v8/v8.git
-	branch = 6.6.164
 	shallow = true
 [submodule "depot_tools"]
 	path = depot_tools