[C++] Remove deprecated functions in 20.0 (C++ subset)

[AlenkaF]

Describe the enhancement requested

Remove subset of deprecated functions listed in #45688:

• GetCapacity in Gandiva cache.cc from GH-41329(Deprecated in 17.0.0)
• BufferReader constructors in io/memory.h from GH-37212 (Deprecated in 14.0.0)
• OpenAppendStream in filesystem/gcsfs.h from ARROW-14222 (Deprecated in 6.0.0)

Component(s)

C++

[pitrou]

Issue resolved by pull request 45748
#45748