nvim: disable beam cursor on insert mode
nvim: disable beam cursor on insert mode

beam cursor in insert mode is set as default by neovim. setting
guicursor to no value fixes this.

Signed-off-by: Ratheesh S <f847af0ee3590f4912d169b886f309989d9e4c2e@gmail.com>
Tested-by: Ratheesh S <f847af0ee3590f4912d169b886f309989d9e4c2e@gmail.com>
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