Remove the mysql2 package peer dependency (#695)

This peer dependency was previously added to fix the build for the
OpenTelemetry mysql2 package instrumentation. In the TypeScript build of
this package it would import the mysql2 types in the
`@opentelemetry/instrumentation-mysql2` package, which we import.

The `@opentelemetry/instrumentation-mysql2` package itself has no
runtime dependency on the mysql2 package, and neither does
`@opentelemetry/auto-instrumentations-node`. Which got me interested to
find out how that package works if we're experiencing the problem
described in PR #688

> Control packages peer dependencies
> We need mysql2 as a peer dependency of the package because the
> OpenTelemetry instrumentation library requires it. This could be fixed
> with introspection using require-in-the-middle.

Instead I've moved it to the devDependencies list to fix the build and
remove the dependency for users that do not use the mysql2 package in
their app.

I've then tested it in a TypeScript example app
appsignal/opentelemetry#29 that doesn't use
mysql2. The AppSignal package works without mysql2 as a peer dependency
in this scenario.

.changesets/remove-mysql2-peer-dependency.md

@@ -0,0 +1,6 @@
+---
+bump: "patch"
+type: "change"
+---
+
+Remove the mysql2 package peer dependency. It's not only present as a development dependency for the package.

package.json

@@ -21,10 +21,8 @@
     "tslib": "^2.0.3",
     "winston": "^3.6.0"
   },
-  "peerDependencies": {
-    "mysql2": "*"
-  },
   "devDependencies": {
+    "mysql2": "*",
     "@types/jest": "^26.0.19",
     "@typescript-eslint/eslint-plugin": "^5.13.0",
     "@typescript-eslint/parser": "^5.13.0",