Revert "feat: add coder_secret data source for user secrets (#501)" (#512)

This reverts commit af77cd5.

Removes the coder_secret data source AND the
SecretEnvEnvironmentVariable / SecretFileEnvironmentVariable helpers
that coderd consumed.

This is being reverted because we are pivoting on the direction of the
secrets feature (see https://linear.app/codercom/issue/PLAT-243).

Assisted by Coder Agents.

Author: zedkipp

docs/data-sources/secret.md

@@ -65,7 +65,6 @@ func New() *schema.Provider {
 			"coder_workspace_owner":  workspaceOwnerDataSource(),
 			"coder_workspace_preset": workspacePresetDataSource(),
 			"coder_task":             taskDatasource(),
-			"coder_secret":           secretDataSource(),
 		},
 		ResourcesMap: map[string]*schema.Resource{
 			"coder_agent":          agentResource(),