Remove perf tests for 5.2 (#1355)

Motivation:

The performance tests have recently become quite unstable for 5.2
and require a few runs to pass. While we could broaden the slack allowed
or investigate further, it doesn't seem worth it: NIO will drop support
for 5.2 when 5.6 is released at which point we'll have to do the same.

Modifications:

- Remove the allocation counting tests for Swift 5.2

Result:

- 5.2 performance is no longer monitored
- CI is less flaky.

.github/workflows/ci.yaml

@@ -81,16 +81,6 @@ jobs:
               MAX_ALLOCS_ALLOWED_unary_1k_ping_pong: 195000
               MAX_ALLOCS_ALLOWED_unary_1k_ping_pong_interceptors_client: 202000
               MAX_ALLOCS_ALLOWED_unary_1k_ping_pong_interceptors_server: 202000
-          - image: swift:5.2-bionic
-            env:
-              MAX_ALLOCS_ALLOWED_bidi_1k_rpcs_10_requests: 492000
-              MAX_ALLOCS_ALLOWED_bidi_1k_rpcs_1_request: 204000
-              MAX_ALLOCS_ALLOWED_embedded_server_bidi_1k_rpcs_10_small_requests: 110000
-              MAX_ALLOCS_ALLOWED_embedded_server_bidi_1k_rpcs_1_small_request: 65000
-              MAX_ALLOCS_ALLOWED_embedded_server_unary_1k_rpcs_1_small_request: 61000
-              MAX_ALLOCS_ALLOWED_unary_1k_ping_pong: 196000
-              MAX_ALLOCS_ALLOWED_unary_1k_ping_pong_interceptors_client: 203000
-              MAX_ALLOCS_ALLOWED_unary_1k_ping_pong_interceptors_server: 203000
     name: Performance Tests on ${{ matrix.image }}
     runs-on: ubuntu-latest
     container: