Removal of Code Supporting Legacy Distros (3.10 tasks)

[dtrudg]

Type of issue
technical debt

Description of issue
SingulartyCE contains various workarounds for RHEL6 / 2.6 kernel, old versions of invoked external programs etc. Special cases supporting these distributions can be removed gradually, but 4.0 is a good target to have removed all to simplify maintenance.

This relates to the host on which Singuarity is installed, not the age of the containerized OS.

• Remove workaround for squashfs-tools < 4.3
• Remove remaining EL6 test conditionals
• No need to handle _LINUX_CAPABILITY_VERSION_2

[dtrudg]

We aren't jumping to a CE 4.0 soon, so this will be taken on as time permits in 3.x milestones.