apacheGH-32801: [C++][Docs] Delete outdated .md files (apache#33829)

### Rationale for this change
According to the issue apache#32801 , there were some outdated files in cpp/apidoc which I deleted.

### What changes are included in this PR?
Deleted Outdated MD Files (HDFS.md plasma.md and tensor_to_py.md)

### Are these changes tested?
No.

### Are there any user-facing changes?
No.

* Closes: apache#32801

Authored-by: Shaheer-Ahmd <shaheerahmad57575@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>