remove now-unused VERSION file, update changelog

9 jobs for v2.1rc1 in 13 minutes and 33 seconds (queued for 12 minutes and 40 seconds)
Status Job ID Name Coverage
  Test
passed #83108
VS2015
VS2015-32-bit

00:01:26

passed #83107
VS2015
VS2015-64-bit

00:01:35

passed #83106
VS2017
VS2017-32-bit

00:01:20

passed #83105
VS2015
VS2017-64-bit

00:01:29

passed #83102
Linux
clang-c++11

00:01:56

passed #83104
Linux
cppcheck

00:03:04

passed #83109
Linux
doc

00:00:10

passed #83101
Linux
gcc-c++11

00:01:53

passed #83103
Apple
macos-c++11

00:01:50