The word you're searching for is "syncope." From Wiki:
In phonology, syncope (/ˈsɪŋkəpiː/; Greek: syn- + koptein "to strike, cut off") is the loss of one or more sounds from the interior of a word, especially the loss of an unstressed vowel. It is found both in synchronic analysis of languages and diachronics. Its opposite, whereby sounds are added, is epenthesis.
As a poetic device
Sounds may be removed from the interior of a word as a rhetorical or poetic device, whether for embellishment or for the sake of the meter.
Latin commo[ve]rat > poetic commorat ("he had moved")
English hast[e]ning > poetic hast'ning
English heav[e]n > poetic heav'n
English over > poetic o'er
English never > poetic ne'er
END WIKI ENTRY
Here are examples in the final two stanzas of Blake's 'Introduction' from "Songs of Innocence" (1789):
Piper, sit thee down and write
In a book, that all may read.
So he vanish'd from my sight,
And I pluck'd a hollow reed,
And I made a rural pen,
And I stain'd the water clear,
And I wrote my happy songs
Every child may joy to hear.