Remove unused CI docker compose files (#6754)

These now exist in the pipelines repo.

.buildkite/docker-compose.py35.pg95.yaml

-version: '3.1'
-
-services:
-
-  postgres:
-    image: postgres:9.5
-    environment:
-      POSTGRES_PASSWORD: postgres
-    command: -c fsync=off
-
-  testenv:
-    image: python:3.5
-    depends_on:
-      - postgres
-    env_file: .env
-    environment:
-      SYNAPSE_POSTGRES_HOST: postgres
-      SYNAPSE_POSTGRES_USER: postgres
-      SYNAPSE_POSTGRES_PASSWORD: postgres
-    working_dir: /src
-    volumes:
-      - ..:/src

.buildkite/docker-compose.py37.pg11.yaml

-version: '3.1'
-
-services:
-
-  postgres:
-    image: postgres:11
-    environment:
-      POSTGRES_PASSWORD: postgres
-    command: -c fsync=off
-
-  testenv:
-    image: python:3.7
-    depends_on:
-      - postgres
-    env_file: .env
-    environment:
-      SYNAPSE_POSTGRES_HOST: postgres
-      SYNAPSE_POSTGRES_USER: postgres
-      SYNAPSE_POSTGRES_PASSWORD: postgres
-    working_dir: /src
-    volumes:
-      - ..:/src

.buildkite/docker-compose.py37.pg95.yaml

-version: '3.1'
-
-services:
-
-  postgres:
-    image: postgres:9.5
-    environment:
-      POSTGRES_PASSWORD: postgres
-    command: -c fsync=off
-
-  testenv:
-    image: python:3.7
-    depends_on:
-      - postgres
-    env_file: .env
-    environment:
-      SYNAPSE_POSTGRES_HOST: postgres
-      SYNAPSE_POSTGRES_USER: postgres
-      SYNAPSE_POSTGRES_PASSWORD: postgres
-    working_dir: /src
-    volumes:
-      - ..:/src

changelog.d/6754.misc

+Remove unused CI docker compose files.