perf: run only with basic vshard

The current state of perf tests is to compare different crud setups
rather than different sharding setups. Current matrix info here is not
explored by anyone for now, but it makes perf tests three times longer
(five times longer with new updates). Since each perf job is already
longer than one hour on CI, I decided to reduce the matrix here to
the most basic case.

test/performance/perf_test.lua

@@ -10,7 +10,7 @@ local t = require('luatest')
 
 local helpers = require('test.helper')
 
-local g = t.group('perf', helpers.backend_matrix())
+local g = t.group('perf', {{backend = helpers.backend.VSHARD}})
 
 local id = 0
 local function gen()