Removed deprecated extend function

[JosefBredereck]

Summary of changes:

The Node Documentation says that util._extend function should not be used.

util._extend(target, source)
Added in: v0.7.5Deprecated since: v6.0.0

target
source

Stability: 0 - Deprecated: Use Object.assign() instead.

The util._extend() method was never intended to be used outside of internal Node.js modules. The community found and used it anyway.

It is deprecated and should not be used in new code. JavaScript comes with very similar built-in functionality through Object.assign().

[stale]

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