Invalid processing of escaped calc() expressions

[aurelien-minvielle]

Given:

calc(~"100% - 4em")

Expected behaviour:

calc(100% - 4em)

Actual behaviour:

calc(~"100% - 4em")

Escaped expressions are no longer needed on recent version of less.js, but are frequent in old stylesheets to workaround old less.js bugs such as this one: less/less.js#974

This would help migration from the old java binding https://github.com/marceloverdijk/lesscss-java to jlessc, as lesscss-java is stucked at less v1.7.0 that requires those calc(~"@{a} - @{b}") constructs.