Update tycho to 2.5.0 and target platform to jgit-4.17

Update tycho to 2.5.0 and target platform to jgit-4.17

jgit-4.17 corresponds to Eclipse 2020-09, which is the first Eclipse
version that required Java 11, and which is JGit's new baseline as of
JGit 6.0.

Remove pack200 which is deprecated in Java 11 and isn't supported by
tycho 2.5.0 anymore. It was specified in JSR 200 (J2SE 1.5) [1],
deprecated in JEP 336 (Java SE 11) [2] and removed in JEP 367 (Java SE
14) [3].

[1] https://www.jcp.org/en/jsr/detail?id=200
[2] https://openjdk.java.net/jeps/336
[3]	https://openjdk.java.net/jeps/367

Change-Id: I12c196586397779d2be81d2374207703e3fc5dbf