Remove `allowSyntheticDefaultImports` in favor of `esModuleInterop` (#7338)

As mentioned on previous PRs related to TypeScript, setting `esModuleInterop`
to `true` automatically sets `allowSyntheticDefaultImports` to `true` as
well, so this config validation is not required.

Reference: https://github.com/zeit/next.js/pull/7169#discussion_r280831148

packages/next/lib/verifyTypeScriptSetup.ts

@@ -107,7 +107,6 @@ export async function verifyTypeScriptSetup(dir: string): Promise<void> {
     lib: { suggested: ['dom', 'dom.iterable', 'esnext'] },
     allowJs: { suggested: true },
     skipLibCheck: { suggested: true },
-    allowSyntheticDefaultImports: { suggested: true },
     strict: { suggested: true },
     forceConsistentCasingInFileNames: { suggested: true },