Remove obsolete mnemonic field from account import state

[csillag]

This is just a left-over field, which was unused;
there ware no security concern, since the data was not stored here anyway.

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ EDITORCONFIG	editorconfig-checker	1		0	0.01s
✅ GIT	git_diff	yes		no	0.01s
✅ TYPESCRIPT	eslint	1	0	0	3.64s

See errors details in artifact MegaLinter reports on CI Job page
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff