Avoid letf macro use from Gnus

Avoid letf macro use from Gnus

gnus/gmm-utils.el (gmm-flet): Remove.
gnus/gnus-sync.el (gnus-sync-lesync-call): Avoid overriding json-alist-p.
gnus/message.el (message-read-from-minibuffer): Avoid overriding mail-abbrev-in-expansion-header-p.
mail/mailabbrev.el (mail-abbrev-expand-wrapper): Work in minibuffer so as to enable message-read-from-minibuffer to expand mail aliases.