buffering: remove bufgettail/bufcuttail

These operations can only be used in limited circumstances and have
exactly one user. bufgettail especially seems of dubious value; how
often do you need to read N bytes from the end of a file without
changing the file position?

strip_tags() was the only function using them, to strip off ID3v1
and APE tags off the end of buffered tracks. This would save only
32-192 bytes per track -- if the container format uses APE/ID3v1.
It hardly seems worth the effort.

Change-Id: I8fc3c1408517eda6126e75e76d76daea904b50eb

docs/PLUGIN_API

@@ -211,13 +211,6 @@ bool bufclose(int handle_id)
     \return
     \description
 
-ssize_t bufcuttail(int handle_id, size_t size)
-    \group buffering API
-    \param handle_id
-    \param size
-    \return
-    \description
-
 ssize_t bufgetdata(int handle_id, size_t size, void **data)
     \group buffering API
     \param handle_id
@@ -226,14 +219,6 @@ ssize_t bufgetdata(int handle_id, size_t size, void **data)
     \return
     \description
 
-ssize_t bufgettail(int handle_id, size_t size, void **data)
-    \group buffering API
-    \param handle_id
-    \param size
-    \param data
-    \return
-    \description
-
 int bufopen(const char *file, size_t offset, enum data_type type)
     \group buffering API
     \param file