common/number: delete unused package

This package was meant to hold an improved 256 bit integer library, but the effort was abandoned in 2015. AFAIK nothing ever used this package. Time to say goodbye.
ethereum · karalabe · Jun 14, 2018 · Jun 14, 2018 · Jun 14, 2018 · f56d4dbc02c6689552bed74178c072506e7393ed
commit f56d4dbc02c6689552bed74178c072506e7393ed
diff --git a/common/number/int.go b/common/number/int.go
diff --git a/common/number/uint_test.go b/common/number/uint_test.go