Remove StringScanner methods absent from the latest Ruby release

StringScanner#clear, #empty?, #getbyte, #peep, and #restsize were
obsolete for over two decades and were removed from strscan 3.1.6,
bundled with the latest Ruby release. RBS targets the latest release
of Ruby, so drop their definitions.

Author: zonuexe

stdlib/strscan/0/string_scanner.rbs

@@ -675,14 +675,6 @@ class StringScanner
   #
   def check_until: (Regexp) -> String
 
-  # <!--
-  #   rdoc-file=ext/strscan/strscan.c
-  #   - clear()
-  # -->
-  # Equivalent to #terminate. This method is obsolete; use #terminate instead.
-  #
-  def clear: () -> void
-
   # <!--
   #   rdoc-file=ext/strscan/strscan.c
   #   - concat(more_string) -> self
@@ -706,14 +698,6 @@ class StringScanner
   #
   alias concat <<
 
-  # <!--
-  #   rdoc-file=ext/strscan/strscan.c
-  #   - empty?()
-  # -->
-  # Equivalent to #eos?. This method is obsolete, use #eos? instead.
-  #
-  def empty?: () -> bool
-
   # <!--
   #   rdoc-file=ext/strscan/strscan.c
   #   - eos? -> true or false
@@ -818,14 +802,6 @@ class StringScanner
   #
   def get_byte: () -> String?
 
-  # <!--
-  #   rdoc-file=ext/strscan/strscan.c
-  #   - getbyte()
-  # -->
-  # Equivalent to #get_byte. This method is obsolete; use #get_byte instead.
-  #
-  def getbyte: () -> String?
-
   # <!--
   #   rdoc-file=ext/strscan/strscan.c
   #   - getch()
@@ -1012,14 +988,6 @@ class StringScanner
   #
   def peek: (Integer) -> String
 
-  # <!--
-  #   rdoc-file=ext/strscan/strscan.c
-  #   - peep(p1)
-  # -->
-  # Equivalent to #peek. This method is obsolete; use #peek instead.
-  #
-  def peep: (Integer) -> String
-
   # <!-- rdoc-file=ext/strscan/strscan.c -->
   # call-seq:
   #  pos -> byte_position
@@ -1217,15 +1185,6 @@ class StringScanner
   #
   def rest_size: () -> Integer
 
-  # <!--
-  #   rdoc-file=ext/strscan/strscan.c
-  #   - restsize()
-  # -->
-  # `s.restsize` is equivalent to `s.rest_size`. This method is obsolete; use
-  # #rest_size instead.
-  #
-  def restsize: () -> Integer
-
   # <!--
   #   rdoc-file=ext/strscan/strscan.c
   #   - scan(p1)