in_tail: remove workaround of converting encoding

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
This PR will remove workarounds for versions earlier than Ruby 2.4.

The following code is causing SEGV in Ruby 2.3.
And I think it was workaround for SEGV.

a = open('/dev/urandom')
buf = ''.force_encoding(Encoding::UTF_16BE)
a.readpartial(2048, buf)

#1409 (comment)

I want to remove it because it works well with currently Ruby.

Docs Changes:

Release Note:

[daipom]

Awesome! Thanks! I will see this soon.

[Watson1978]

Hmm, this fails CI...
I will look test codes...

[Watson1978]

Hmm, this PR might cause a Encoding::CompatibilityError when append the chunk data...

>> @buffer = ''.force_encoding(Encoding::UTF_16LE)
=> ""
>> chunk = 'あいうえお'.force_encoding('ASCII-8BIT')
=> "\xE3\x81\x82\xE3\x81\x84\xE3\x81\x86\xE3\x81\x88\xE3\x81\x8A"
>> @buffer << chunk
=> "\xE3\x81\x82\xE3\x81\x84\xE3\x81\x86\xE3\x81\x88\xE3\x81\x8A"
>> @buffer.encoding
=> #<Encoding:BINARY (ASCII-8BIT)>
>> @buffer.force_encoding(Encoding::UTF_16LE)
=> "\u81E3\uE382\u8481\u81E3\uE386\u8881\u81E3\x8A"
>> @buffer << chunk
(irb):7:in 'String#<<': incompatible character encodings: UTF-16LE and BINARY (ASCII-8BIT) (Encoding::CompatibilityError)
	from (irb):7:in '<main>'
	from <internal:kernel>:168:in 'Kernel#loop'
	from /home/watson/.rbenv/versions/3.4.1/lib/ruby/gems/3.4.0/gems/irb-1.15.1/exe/irb:9:in '<top (required)>'
	from /home/watson/.rbenv/versions/3.4.1/bin/irb:25:in 'Kernel#load'
	from /home/watson/.rbenv/versions/3.4.1/bin/irb:25:in '<main>'

[daipom]

I see.
Thanks for this endeavor.
Let us know if you find out anything about this.
Thanks!