How to unmap all combinations that begin with ?

I found these default mappings after typing :imap

i <Esc>[6~ <PageDown>
i <Esc>[5~ <PageUp>
i <Esc>OF <End>
i <Esc>OH <Home>
i <Esc>OD <Left>
i <Esc>OC <Right>
i <Esc>OB <Down>
i <Esc>OA <Up>

I want to get rid of all these mappings in my vimrc, so I can properly
map key sequences that begin with escape without having vim wait to
leav insert mode. I have tried to put the following in my vimrc:

iunmap <Esc>[6~
iunmap <Esc>[5~
iunmap <Esc>OF
iunmap <Esc>OH
iunmap <Esc>OD
iunmap <Esc>OC
iunmap <Esc>OB
iunmap <Esc>OA

But it doesn't work and vim complains with messages of 'no such mappings'

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