gnu: Remove unneeded uses of 'libiconv'.

gnu: Remove unneeded uses of 'libiconv'.

iconv is part of glibc, there is no need to add it as input.

* gnu/packages/accessibility.scm (brltty)[inputs]: Remove LIBICONV.
* gnu/packages/compression.scm (innoextract)[inputs]: Likewise.
* gnu/packages/haskell-apps.scm (darcs)[inputs]: Likewise.
* gnu/packages/kde-multimedia.scm (k3b)[inputs]: Likewise.
* gnu/packages/linphone.scm (liblinphone)[inputs]: Likewise.
* gnu/packages/mail.scm (mailutils)[inputs]: Likewise.
* gnu/packages/maths.scm (libqalculate)[inputs]: Likewise.
* gnu/packages/pep.scm (pep-engine)[inputs]: Likewise.
* gnu/packages/vim.scm (neovim)[inputs]: Likewise.
* gnu/packages/wget.scm (wget2)[inputs]: Likewise.
* gnu/packages/telegram.scm (telegram-desktop)[inputs]: Likewise.
* gnu/packages/video.scm (transcode)[native-inputs]: Likewise.