Remove CircleCI because it does not support macOS on free plan

.circleci/config.yml

@@ -1,26 +0,0 @@
-version: 2
-jobs:
-  build:
-    macos:
-      xcode: "10.2.0"
-
-    steps:
-      - checkout
-      - run:
-          name: Install dependencies
-          command: brew install go qemu
-      - run:
-          name: Build
-          command: go build
-      - run:
-          name: End-to-End Testing [Kernel Module]
-          command: |
-            cd examples/kernel-module
-            ../../out-of-tree kernel autogen --max=1
-            ../../out-of-tree pew --qemu-timeout=10m
-      - run:
-          name: End-to-End Testing [Kernel Exploit]
-          command: |
-            cd examples/kernel-exploit
-            ../../out-of-tree kernel autogen --max=1
-            ../../out-of-tree pew --threshold=0 --qemu-timeout=10m