Although I strongly recommend switching over to using the package manager version of TextMesh Pro which will require converting the project over to this new version. The package manager version of TMP does include source code which is great. See the following thread with information about this.
If you do choose to migrate to the new version, make sure you grab the latest which is 1.2.2. Version 1.2.3 should be release soon as well.
Having said that, if you want to continue using the previous version of TMP, you can prevent the package version from being installed by modifying the “Project Root/Packages/manifest.json” to include the following
{
"dependencies": {
"com.unity.textmeshpro": "exclude"
}
}