How can I remove line annotations behind cursor in Intellij 2019.1?

That isn't an IntellIJ IDEA feature.
You installed the GitToolBox plugin.

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This is called Git blame.
You enable it by right-clicking the gutter area (if you configured a valid Git root)

And you disable it in the same way with Close Annotations

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The displayed information can be altered using the View sub-menu

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All the necessary IDEA examples/explanation can be found here.
For your case, under the Locate code author paragraph.

Newer versions of intellij has a feature called "Inlay Hints". Open preferences and search for it. Uncheck those you don't want to see:

You have installed GitToolBox third-party plug-in which provides this feature:

Blame inlined in active editor - show blame for line at caret in active editor

Either disable the plug-in or configure its options as shown below: